Plasma etching of Ni-containing materials

ABSTRACT

An apparatus and method are described for etching Ni-containing films using gas phase plasma etching. Etching of Ti—Ni alloys is carried out by exposure to plasma comprising hydrogen halide (HX) and carbonyl etching gases. The Ti in the Ti—Ni alloy is etched via an ion-assisted reaction with HX and the Ni is etched by reacting with CO. The method is particularly well suited for anisotropic etching of Ti—Ni metal gates for CMOS applications. Etching of Ni—Fe layers is carried out by exposure to plasma comprising a carbonyl etching gas.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional Ser. No.60/352,206, filed on Jan. 29, 2002, the entire contents of which areherein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to gas phase plasma etching of Nickel (Ni)containing materials used in semiconductor applications.

BACKGROUND OF THE INVENTION

In the semiconductor industry, the minimum feature sizes ofmicroelectronic devices are approaching the deep sub-micron regime tomeet the demand for faster, lower power microprocessors and digitalcircuits. Process development and integration issues are key challengesfor new gate stack materials and silicide processing, where the imminentreplacement of SiO₂ with higher permittivity dielectric materialsnecessitates the use of alternative gate electrode materials to replacedoped poly-silicon.

The introduction of metal gate electrodes to replace the traditionalpoly-silicon gates brings about several advantages. These advantagesinclude elimination of the poly-silicon gate depletion effect, reductionin sheet resistance, better reliability and potentially better thermalstability on advanced high-permittivity gate dielectrics. One of thematerial selection criteria for the metal gate electrode is that theworkfunction be adjustable. Control over the gate workfunction isachieved by using a dual metal gate, since a metal alloy containingvariable amounts of the metal constituents can yield an adjustableintermediate gate work function. From a processing point of view, themetal gate should be easily deposited by conventional techniques, suchas CVD or sputtering, and be selectively etched by commonly used plasmaetching processes. However, there are a number of processingdifficulties in the dual metal gate approach. Etching characteristics ofthe different metal components are likely to be different, which canlead to etch rates that vary greatly from one metal to another,resulting in poor edge profiles and the need for complex manufacturingprocesses. Capacitively coupled plasma sources are widely used for dryetching processes to remove a layer of material from a wafer surface. Atthese small dimensions, etch uniformity and selectivity over the wafersurface has become increasingly more important, particularly when alayer is being etched according to a pattern. When manufacturingcircuits with advanced feature sizes, anisotropic etching methods areneeded that allow excellent control over the etching process. This isachieved using ion-assisted etching processes that comprise a strongphysical component in addition to a chemical component. The chemicalcomponent conventionally involves a mixture of process gases that reactwith the substrate and form a reaction layer on the substrate. Thephysical component involves subsequent direct line-of-sight impaction ofgas phase ions on the surface that results in physical desorption of thereaction products from the surface and material removal. The reaction(passivation) layer on the sidewalls of high-aspect-ratio structuresreceives low levels of ion bombardment, resulting in slow etching of thesidewalls, which leads to anisotropic etching profiles.

Potential Ni-containing materials for use as metal gates include Ti—Nidual metal alloys. Ti—Ni alloys are known to be thermally stable, whichallows them to be used at the high temperatures that are necessary forsource/drain implant activation. Etching of Ti- and Ni-containingmaterials has been carried out using halogen-based chemistry, buthalogen plasma etching of Ni is difficult, in part since Ni-halideetching products (e.g., NiBr₂, NiCl₂ or Nil₂) have very low vaporpressures and high process temperatures (>200° C.) are needed forion-assisted etching of the Ni-halide reaction products. The hightemperature requirement along with potential redeposition of theNi-halide etching products on to the chamber hardware imposes seriousrestrictions on the reliability and productivity of the etching chamberhardware. Removal of Ni-halide etching products is possible at lowtemperatures through physical sputtering, but this method is undesirablefor semiconductor processing and integration into gate-conductoretching.

These problems illustrate that successful integration of Ni-containingmetal gates into conventional CMOS technology requires ion-assistedplasma etching methods that allow anisotropic etching of the metal gatesat low temperatures. In addition, it is important that these etchingmethods result in etching products that do not redeposit on the walls ofthe processing chamber.

Other Ni-containing materials that require low temperature etchingmethods include Ni—Fe layers which are important in magnetoresistiverandom access memory (MRAM) devices.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a plasmaprocessing system and method for low temperature etching of aNi-containing layer for manufacturing an integrated circuit. The processenvironment comprises process gases that are capable of anisotropicetching of high-aspect-ratio features.

A further object of the invention is to provide a method of etching aTi—Ni layer using a gaseous plasma environment comprising hydrogenhalide (HX) and carbonyl process gases.

The above and other objects are achieved, according to the presentinvention, by providing an apparatus and a method that uses a gaseousplasma environment comprising HX and carbonyl process gases. Anickel-containing layer on a substrate is exposed to plasma containinghydrogen halide and carbonyl process gases. The HX etching gas reactswith Ti in the metal alloy, forming TiX_(n) reaction products that areremoved from the substrate as a result of ion-assisted etching, and thecarbonyl process gas reacts with the Ni in the metal alloy, forming avolatile nickel carbonyl (Ni(CO)₄) reaction product that is removed fromthe substrate. A passivating layer of TiX_(n) is formed on featuresidewalls, which allows anisotropic etching in accordance with a maskpattern.

A further object of the invention is to provide a method for etching aNi—Fe layer using a gaseous plasma environment comprising a carbonylprocess gas.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will become readily apparent with reference to thefollowing detailed description, particularly when considered inconjunction with the accompanying drawings, in which:

FIGS. 1 a–1 c show a schematic cross-sectional representation of aplasma etching method in accordance with the present invention;

FIG. 2 is a flowchart for etching a Ni-containing layer according thepresent invention;

FIG. 3 shows a plasma processing system according to a preferredembodiment of the present invention;

FIG. 4 shows a plasma processing system according to an alternateembodiment of the present invention;

FIG. 5 shows a plasma processing system according to an alternateembodiment of the present invention;

FIG. 6 shows a plasma processing system according to an alternateembodiment of the present invention; and

FIG. 7 shows a plasma processing system according to an alternateembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention pertains to a plasma processing methodfor etching a Ni-containing layer for manufacturing an integratedcircuit.

In one embodiment, an apparatus and a method are provided foranisotropic etching of Ni-containing structures using a mixture ofprocess gases. In the case of Ti—Ni alloys, anisotropic plasma etchingin accordance with a mask pattern is achieved at low temperatures usinga gaseous mixture comprising HX (where X can be a halide) and carbonyl(e.g., CO and CO₂) process gases.

The otherwise slow rates of removal of Ni-halides during plasma etchingof Ti—Ni alloys using etch gases comprising halogens is addressed byintroducing a mixture of etching gases that comprise HX and carbonylgases. These gases are mostly non-reactive towards each other but reacteffectively with the components of the metal alloy, resulting in fastanisotropic etching.

FIGS. 1 a–1 c show a schematic cross-sectional representation of aplasma etching process in accordance with the present invention.

FIG. 1 a shows a partially completed integrated circuit. The segment 100comprises a bulk-Si substrate 118, a layer of gate dielectric 116, ametal gate layer 110, an anti-reflection coating 114, and a photoresistpattern 112. Anisotropic etching of the structure in FIG. 1 a accordingto the present invention etches the antireflective coating 114 and themetal alloy 110 while preserving the vertical geometry of the structuredefined by the photoresist pattern 112, forming the structure shown inFIG. 1 b. Referring to FIG. 1 b, continued processing as is conventionalin the art, removes the remaining photoresist pattern 112 and theantireflective coating 114, resulting in the structure shown in FIG. 1c.

A Ti—Ni metal gate layer is etched according to the present inventionusing HX and carbonyl process gases. The material removal proceeds viaion-assisted etching of Ti-halide (TiX_(n)) etch products and thermaldesorption of nickel carbonyl (Ni(CO)₄) etch products. Duringanisotropic etching of the circuit features depicted in FIG. 1 a, apassivating layer 120 containing an adsorbed TiX_(n) reaction layer isformed on the side-walls of the partially etched Ti—Ni feature 110,where the ion-assisted etching of TiX_(n) is very slow due to nearabsence of ion bombardment. The passivating layer 120 blocks furtherreaction of Ti and Ni on the sidewalls and leads to anisotropic etchingof the feature 110. On horizontal surfaces depicted in FIG. 1 a, theetching proceeds via ion-assisted TiX_(n) etching and formation ofvolatile Ni(CO)₄.

In a preferred embodiment, the process gas includes a first gascontaining a hydrogen halide and a second gas containing carbonyl gas.The hydrogen halide can be selected from the group containing hydrogenbromide (HBr), hydrogen chloride (HCl), and hydrogen iodide (HI). Thecarbonyl gas can be selected from the group containing carbon monoxide(CO) and carbon dioxide (CO₂).

The choice of halide for the HX process gas depends on plasma reactortype and process conditions. The HX must form a TiX_(n) reaction layerthat is etchable by ion-assisted desorption at a preferred temperature.For a given HX gas, if the process temperature is too low, excesspassivation of the sidewall by TiX_(n) can occur and lead to a taperedprofile. If the process temperature is too high, the TiX_(n) can desorbin the absence of ion bombardment from sidewall surfaces and this canlead to unacceptable undercutting of circuit features. HCl and HBrprocess gases have been shown to work well for substrate temperatures ofabout 80° C., but HI can require higher temperatures because the Til_(n)reaction product is less volatile than TiBr_(n) or TiCl_(n).

Dissociation of CO₂ into CO is followed by reaction of CO with Ni in theNi-containing layer being etched. Embodiments using CO instead of CO₂follow direct reaction with Ni without the CO₂ dissociation step.

In an alternate embodiment, an inert gas is added to any one of theaforementioned process gas chemistries. The inert gas may include atleast one of argon, helium, xenon, krypton and nitrogen. For example,the addition of inert gas to the process chemistry is used to dilute theprocess gas or adjust the process gas partial pressure(s). Furthermore,for example, the addition of inert gas can aid the physical component ofthe feature etch.

Flow rates of the hydrogen halide and carbonyl gas can be independentlycontrolled. Exemplary flow rates for each are from 0 to 1000 sccm, withtypical values being less than 500 sccm and preferably between 1 and 500sccm.

In an alternate embodiment of the present invention, Ni—Fe layers areetched in a plasma system comprising a carbonyl process gas. Etchingproceeds via formation of volatile Ni(CO)₄ and Fe(CO)₅ etch products. Aninert gas comprising at least one of argon, helium, xenon, krypton andnitrogen can be included in the process gas.

FIG. 2 is a flowchart for etching a Ni-containing layer according thepresent invention. Step 200 provides a Ni-containing layer to be etchedin a plasma process chamber. Process gases are introduced to the processchamber in step 210 and plasma is formed in step 220. The Ni-containinglayer is exposed to the plasma in step 230 for a time period thatenables etching of the Ni-containing layer.

FIG. 3 shows a plasma processing system according to a preferredembodiment of the present invention. A plasma processing system 1 thatis capable of sustaining a plasma is depicted in FIG. 3, which includesa plasma process chamber 10, a substrate holder 20, upon which asubstrate 25 to be processed is affixed, and a gas injection system 40for introducing process gases to the plasma process chamber 10.

FIG. 4 shows a plasma processing system according to an alternateembodiment of the present invention. A plasma processing device 1 isdepicted which includes a chamber 10, a substrate holder 20, upon whicha substrate 25 to be processed is affixed, a gas injection system 40,and a vacuum pumping system 50. Chamber 10 is configured to facilitatethe generation of plasma in a processing region 45 adjacent a surface ofsubstrate 25, wherein plasma is formed via collisions between heatedelectrons and an ionizable gas. An ionizable gas or mixture of gases isintroduced via the gas injection system 40 and the process pressure isadjusted. For example, a gate valve (not shown) is used to throttle thevacuum pumping system 50. Desirably, plasma is utilized to creatematerials specific to a pre-determined materials process, and to aideither the deposition of material to a substrate 25 or the removal ofmaterial from the exposed surfaces of the substrate 25.

Substrate 25 is transferred into and out of chamber 10 through a slotvalve (not shown) and chamber feed-through (not shown) via roboticsubstrate transfer system where it is received by substrate lift pins(not shown) housed within substrate holder 20 and mechanicallytranslated by devices housed therein. Once the substrate 25 is receivedfrom the substrate transfer system, it is lowered to an upper surface ofthe substrate holder 20.

In an alternate embodiment, the substrate 25 is affixed to the substrateholder 20 via an electrostatic clamp (not shown). Furthermore, thesubstrate holder 20 further includes a cooling system including are-circulating coolant flow that receives heat from the substrate holder20 and transfers heat to a heat exchanger system (not shown), or whenheating, transfers heat from the heat exchanger system. Moreover, gasmay be delivered to the backside of the substrate to improve the gas-gapthermal conductance between the substrate 25 and the substrate holder20. Such a system is utilized when temperature control of the substrateis required at elevated or reduced temperatures. For example,temperature control of the substrate may be useful at temperatures inexcess of the steady-state temperature achieved due to a balance of theheat flux delivered to the substrate 25 from the plasma and the heatflux removed from substrate 25 by conduction to the substrate holder 20.In other embodiments, heating elements, such as resistive heatingelements, or thermo-electric heaters/coolers are included.

In the embodiment, shown in FIG. 4, the substrate holder 20 furtherserves as an electrode through which radio frequency (RF) power iscoupled to plasma in the processing region 45. For example, thesubstrate holder 20 is electrically biased at a RF voltage via thetransmission of RF power from an RF generator 30 through an impedancematch network 32 to the substrate holder 20. The RF bias serves to heatelectrons and, thereby, form and maintain plasma. In this configuration,the system operates as a reactive ion etch (RIE) reactor, wherein thechamber and upper gas injection electrode serve as ground surfaces. Atypical frequency for the RF bias ranges from 1 MHz to 100 MHz and ispreferably 13.56 MHz.

In an alternate embodiment, RF power is applied to the substrate holderelectrode at multiple frequencies. Furthermore, the impedance matchnetwork 32 serves to maximize the transfer of RF power to plasma inprocessing chamber 10 by minimizing the reflected power. Match networktopologies (e.g. L-type, π-type, T-type) and automatic control methodsare known in the art.

With continuing reference to FIG. 4, a process gas 42 is introduced tothe processing region 45 through the gas injection system 40. Gasinjection system 40 can include a showerhead, wherein the process gas 42is supplied from a gas delivery system (not shown) to the processingregion 45 through a gas injection plenum (not shown), a series of baffleplates (not shown) and a multi-orifice showerhead gas injection plate(not shown).

Vacuum pump system 50 preferably includes a turbo-molecular vacuum pump(TMP) capable of a pumping speed up to 5000 liters per second (andgreater) and a gate valve for throttling the chamber pressure. Inconventional plasma processing devices utilized for dry plasma etch, a1000 to 3000 liter per second TMP is employed. TMPs are useful for lowpressure processing, typically less than 50 mTorr. At higher pressures,the TMP pumping speed falls off dramatically. For high pressureprocessing (i.e. greater than 100 mTorr), a mechanical booster pump anddry roughing pump are used.

A controller 55 includes a microprocessor, a memory, and a digital I/Oport capable of generating control voltages sufficient to communicateand activate inputs to the plasma processing system 1 as well as monitoroutputs from the plasma processing system 1. Moreover, the controller 55is coupled to and exchanges information with the RF generator 30, theimpedance match network 32, the gas injection system 40 and the vacuumpump system 50. A program stored in the memory is utilized to controlthe aforementioned components of a plasma processing system 1 accordingto a stored process recipe. One example of controller 55 is a DELLPRECISION WORKSTATION 610™, available from Dell Corporation, Dallas,Tex.

FIG. 5 shows a plasma processing system according to an alternateembodiment of the present invention. The plasma processing system 1further includes either a mechanically or electrically rotating DCmagnetic field system 60, in order to potentially increase plasmadensity and/or improve plasma processing uniformity, in addition tothose components described with reference to FIG. 4. Moreover, thecontroller 55 is coupled to the rotating magnetic field system 60 inorder to regulate the speed of rotation and field strength.

FIG. 6 shows a plasma processing system according to an alternateembodiment of the present invention. The plasma processing system 1 ofFIG. 4 further includes an upper plate electrode 70 to which RF power iscoupled from an RF generator 72 through an impedance match network 74. Atypical frequency for the application of RF power to the upper electroderanges from 10 MHz to 200 MHz and is preferably 60 MHz. Additionally, atypical frequency for the application of power to the lower electroderanges from 0.1 MHz to 30 MHz and is preferably 2 MHz. Moreover, thecontroller 55 is coupled to the RF generator 72 and the impedance matchnetwork 74 in order to control the application of RF power to the upperelectrode 70.

FIG. 7 shows a plasma processing system according to an alternateembodiment of the present invention. The plasma processing system ofFIG. 4 is modified to further include an inductive coil 80 to which RFpower is coupled via an RF generator 82 through an impedance matchnetwork 84. RF power is inductively coupled from the inductive coil 80through a dielectric window (not shown) to the plasma processing region45. A typical frequency for the application of RF power to the inductivecoil 80 ranges from 10 MHz to 100 MHz and is preferably 13.56 MHz.Similarly, a typical frequency for the application of power to the chuckelectrode ranges from 0.1 MHz to 30 MHz and is preferably 13.56 MHz. Inaddition, a slotted Faraday shield (not shown) is employed to reducecapacitive coupling between the inductive coil 80 and plasma. Moreover,the controller 55 is coupled to the RF generator 82 and the impedancematch network 84 in order to control the application of power to theinductive coil 80.

In an alternate embodiment, the plasma is formed using electroncyclotron resonance (ECR). In yet another embodiment, the plasma isformed from the launching of a Helicon wave. In yet another embodiment,the plasma is formed from a propagating surface wave.

It should be understood that various modifications and variations of thepresent invention may be employed in practicing the invention. It istherefore to be understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

1. A method of processing a nickel-containing layer comprising:providing a nickel-containing layer overlying a substrate; introducing aprocess gas, said process gas comprising a carbonyl gas and a hydrogenhalide gas; forming plasma from said process gas; and etching saidnickel-containing layer by exposing said nickel-containing layer to saidplasma, wherein said process gas reacts with said nickel-containinglayer and etches completely through a portion of said nickel-containinglayer to said substrate.
 2. The method according to claim 1, whereinsaid substrate is maintained at a temperature of between 40° C. and 100°C.
 3. The method according to claim 1, wherein said hydrogen halidecomprises at least one of hydrogen bromide (HBr), hydrogen chloride(HCl) and hydrogen iodide (HI).
 4. The method according to claim 1,wherein said carbonyl gas comprises at least one of carbon monoxide (CO)and carbon dioxide (CO₂).
 5. The method according to claim 1, whereinsaid nickel-containing layer contains nickel and titanium.
 6. The methodaccording to claim 1, wherein said nickel-containing layer containsnickel and iron.
 7. The method according to claim 1, wherein saidprocess gas comprises HBr and CO.
 8. The method according to claim 7,wherein a flowrate of HBr is less than 500 sccm and a flowrate of CO isless than 500 sccm.
 9. The method according to claim 1, wherein saidprocess gas comprises HBr and CO₂.
 10. The method according to claim 9,wherein a flowrate of HBr is less than 500 sccm and a flowrate of CO₂ isless than 500 sccm.
 11. The method according to claim 1, wherein saidprocess gas comprises HCl and CO.
 12. The method according to claim 11,wherein a flowrate of HCl is less than 500 sccm and a flowrate of CO isless than 500 sccm.
 13. The method according to claim 1, wherein saidprocess gas comprises HCI and CO₂.
 14. The method according to claim 13,wherein a flowrate of HCl is less than 500 sccm and a flowrate of CO₂ isless than 500 sccm.
 15. The method according to claim 1, wherein saidprocess gas comprises HI and CO.
 16. The method according to claim 15,wherein a flowrate of HI is less than 500 sccm and a flowrate of CO isless than 500 sccm.
 17. The method according to claim 1, wherein saidprocess gas comprises HI and CO₂.
 18. The method according to claim 17,wherein a flowrate of HI is less than 500 sccm and a flowrate of CO₂ isless than 500 sccm.
 19. The method according to claim 1, wherein saidprocess gas also comprises an inert gas.
 20. The method according toclaim 19, wherein said inert gas comprises at least one of argon,helium, xenon and nitrogen.